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The effect of glass composition on the mean dispersion of barium borate glasses
Journal of Non-Crystalline Solids 29 (1978) 231-237 © North-Holland Publishing Company
THE EFFECT OF GLASS COMPOSITION ON THE MEAN DISPERSION OF BARIUM BORATE GLASSES D. WARNER and H. RAWSON Department o f Ceramics, Glasses and Polymers, University o f Shef~eld, UK
Received 8 November 1977 Revised manuscript received 18 January 1978
The mean dispersion has been determined for glasses in the BaO-B203 system containing between 20 and 40 mol% BaO. The results could be plotted as two straight lines intersecting at about 33 mol% BaO. However, statistical analysis of the results shows that a smooth curve could be drawn to fit the results equally well. Other properties, especially molar refractivity, show marked changes in the vicinity of 33 mol% BaO. The results are discussed in the light of structural information obtained from NMR studies.
1. Introduction Hamilton et al. [1] have studied the effect of chemical composition on the refractive index and dispersion of glasses in the BaO--BzO3-SiO 2 system. They report the values of the refractive index nD and the Abbe number v = ( n o - 1)/ (nF" -- no), from which it is possible to calculate the mean dispersion nF" - no. The values so obtained are plotted in fig. 1. There is a suggestion that the results may be represented by two straight lines with a change in slope at approximately 32 mol% BaO. In silicate systems, e.g. Na20-SiO2, the mean dispersion increases linearly with increasing alkali content. The literature on glass properties contains many observations of p r o p e r t y - c o m position relationships showing maxima, minima or changes in slope. When changes in slope are reported, there is always a suspicion that the results might equally well be represented by a smooth curve. Also, it is not often possible to relate convincingly the position of the change in slope with what is known about the structure o f the glass. For glasses in the B a O - B 2 0 3 system, Greenblatt and Bray [2] maintain, on the basis of their interpretation of NMR results, that, in the composition range below ~33 mol% BaO, the oxygen ions introduced by the BaO are taken up by the formation of BO 4 tetrahedra. At higher BaO contents, the fraction of four-coordinated borons was found to decrease. Although the situation is not absolutely clear-cut, in that the glasses studied contained between 6 and 12% silica, it is not unreasonable 231
232
D. lYarner, H. Rawson / Mean dispersion of barium borate glasses
to expect, on the basis of the results of Greenblatt and Bray, that non-bridging oxygens begin to appear beyond 33 tool% BaO. Stevels [3] and many subsequent authors have proposed that the UV cutoff in oxide glasses is associated with the excitation of electrons in the oxygen ions. It is also suggested that the excitation energy is less for a non-bridging than for a bridging oxygen ion. This explains the shift of the UV absorption edge of silicate glasses to longer wavelengths with increasing content of basic oxide. Because of the connection between absorption and dispersion, one expects the dispersion to increase with increasing content of basic oxide. One would also expect to find a more rapid rate of increase of dispersion if the basic oxide addition results in an increasing proportion o f non-bridging oxygens than if it does not. In the light of these considerations, the increase in slope of the dispersion-composition curve at about 33 tool% BaO is not surprising. However, the method used for obtaining the mean dispersions from the data of Hamilton et al. gives values the accuracy of which it is difficult to assess. The error on individual values of the Abbe number was expected to be as much as +1. Thus, the calculated mean dispersion values could be between 1 and 2% in error. The purpose of the present work was to obtain more data so as to be able to test statistically for the existence of a discontinuity in the dispersion-composition relationship.
2. Experimental A series of barium borate glasses was melted from A.R. grade chemicals in the range 2 0 - 4 0 tool%. The glasses were analysed, the results being given in table 1. A
Table 1 Glass compositions determined by analysis. mol%
1 2 3 4 5 6 7 8 9
BaO
B203
20.7 23.3 26.0 28.5 30.8 33.4 35.8 38.6 40.0
79.3 76.7 74.0 71.5 69.2 66.6 64.2 61.4 60.0
D. Warner, tt. Rawson /Mean dispersion of barium borate glasses
233
duplicate analysis was carried out on glass 9 only. These gave BaO contents of 40.05 and 40.08%. Hamilton et al. consider that the probable error in their BaO values is -+0.06 tool%. The glasses were prepared in a platinum alloy crucible and were homogenized by first bubbling oxygen through the melt and then stirring mechanically, using a motor-driven platinum blade mounted on the end of a refractory rod. The temperature was varied between 1050 and 1200°C during melting to obtain optimum homogenization and refining. The melting time was about 7 h. The melt was cast into metal moulds, care being taken to prevent folding of the glass stream as it flowed fr from the crucible into the mould. The moulds were preheated to 3 0 0 - 3 5 0 ° C , otherwise the castings cracked. The blocks were roughly annealed by placing them in a muffle furnace held at 610°C. The furnace was allowed to cool freely for 16 h to room temperature. The blocks were cut into rectangular samples suitable for refractive index measurement. After grinding and polishing these measured approximately 15 X 15 X 5 nun. They were then given a more careful anneal. [n the cylindrical tube-furnace used, each specimen was placed in a hole in a thick, circular, stainless steel plate, the holes being equidistant from the central vertical axis of the furnace. The temperature was raised to 620°C and held for 6 rain, after which the power was switched off. The cooling rate down to 500°C was almost constant at about 2.5 ° rain - l . A Pulfrich refractometer was used to measure the refractive indices for the sodium D line and the hydrogen C and F lines. At each wavelength 10 readings were made on each specimen, the specimen being removed from the instrument prism and replaced between each measurement. The optical sealing liquid used was ~-monobromonaphthalene. Care was taken to minimize the amount of liquid used so that the liquid film between the specimen and the instrument prism was as thin as possible. The density of each specimen was measured by the Archimedes method. The specimens, which were free from any seed which could be detected by eye, were weighed in air and toluene.
3. Results The density and refractive index results are given in table 2. Fig. 1 compares the mean dispersion results of the present work with those of Hamilton et al. The length of the vertical lines corresponds to two standard deviations. Regression analyses were carried out to determine whether the results could be represented by: (a) a straight line, (b) two straight lines of different slopes intersecting at 33 tool% BaO, (c) a smooth curve of the f o r m y = ax b. The results in fable 3 shows lhat there is nothing to choose between hypotheses (b) and (c). The density results are shown in fig. 2. Below 31 tool% BaO the results fall almost exactly on a straight line. Beyond this point the values fall below the line obtained by extrapolating that
234
D. Warner,H. Rawson /Mean dispersion of barium borate glasses
¢)
o
.=.
D. Warner, H. Rawson /Mean dispersion o f barium borate glasses
235
11 0 x 10 -3
10.5
/ //
ill 17 z
o
100
u') c3
95
90
8 5
210
I 25
I 30
t 35
I 40
BoO CONTENT, mol. %
Fig. 1. Relationship between mean dispersion and composition of glasses in the B a O - B 2 0 3 system. Solid circles are the results of Hamilton et al. [ 1 ] ; open circles are those from the present work.
p l o t t e d using t h e similarly. Finally, fig. 3 sition. B o t h o u r a b o u t 32 tool%
results at lower BaO c o n t e n t s . T h e refractive i n d e x results b e h a v e s h o w s the r e l a t i o n s h i p b e t w e e n m o l a r r e f r a c t i v i t y R m a n d c o m p o results a n d t h o s e o f H a m i l t o n et al. s h o w s a b r o a d m i n i m u m at BaO. T h e m o l a r r e f r a c t i v i t y was c a l c u l a t e d using t h e e q u a t i o n
R m = (/7 2 -- l ) / ( r / 2 + 2 ) .
Vm,
w h e r e F m is the m o l a r v o l u m e .
Table 3 Linear regression analysis of mean dispersion results. Correlation coefficient 1. Two straight lines intersecting at 33 mol% BaO 2. Smooth curve of the form y = ax b
0.981 0.984
D. Warner, H. Rawson /Mean dispersion of barium borate glasses
236
i
i
i
3.8
/ //
3.6
'7 ~ E
34
)FN z w o
3.2
3(
28
20
I
I
I
25
30
35
40
BQO CONTENT, tool %
Fig. 2. Relationship b e t w e e n density and c o m p o s i t i o n of glasses in the B a O - B 2 0 3 system.
990
,
r
,
i 30
L 35
\\
u >_" >
i~
985
0
9.80
9 75
i 20
i 25
i 40
BoO CONTENT, mol. % Fig. 3. Relationship b e t w e e n molar refractivity and c o m p o s i t i o n o f glasses in the B a O - B 2 0 3 system. Solid circles are the results of H a m i l t o n et al. [1 ]; o p e n circles are those f r o m the present work.
D. Warner, H. Rawson / Mean dispersion of barium borate glasses
237
4. Conclusions Although the dispersion results obtained in the present work do not provide any move support for the existence of a discontinuity in the dispersion-composition relationship than do the results of Hamilton et al.; the trends of other properties show significant changes at about 33 mol% BaO. These changes eorrelate well with changes indicated by a technique for studying local structure, i.e. NMR, which is reasonably well established. Although the molar refractivity versus composition plot provides the most obvious indication that something significant seems to be happening to the glass structure at about 33 mol% BaO, it is our opinion that, when considering how this work might be extended, it would be preferable to concentrate on the dispersion-composition relationship in that the interpretation of the phenomenon of dispersion is more straightforward than the interpretation of density and refractive index results. Also, the dispersion is a property related to the same structural feature as that detectable by NMR, i.e. the proportion of non-bridging oxygens. It would be of interest to repeat the present work using a better instrument for measuring refractive index than the one available to us, and also to study dispersion-composition relationships in other systems on which NMR studies have been carried out, e.g. the NazOB203-SiO 2 system.
References [ 1] E.H. ttamilton, G.W. Cleek and O.H. Grauer, J. Am. Ceram. Soc. 41 (1958) 209-215. [2] S. Greenblatt and P.J, Bray, Phys. Chem. Glasses 8 (1967) 190-193. [3] J.M. Stevels, Proc. 1lth Congr. Pure and Appl. Chem., Vol. 5 (1953) pp. 519-522.
THE EFFECT OF GLASS COMPOSITION ON THE MEAN DISPERSION OF BARIUM BORATE GLASSES D. WARNER and H. RAWSON Department o f Ceramics, Glasses and Polymers, University o f Shef~eld, UK
Received 8 November 1977 Revised manuscript received 18 January 1978
The mean dispersion has been determined for glasses in the BaO-B203 system containing between 20 and 40 mol% BaO. The results could be plotted as two straight lines intersecting at about 33 mol% BaO. However, statistical analysis of the results shows that a smooth curve could be drawn to fit the results equally well. Other properties, especially molar refractivity, show marked changes in the vicinity of 33 mol% BaO. The results are discussed in the light of structural information obtained from NMR studies.
1. Introduction Hamilton et al. [1] have studied the effect of chemical composition on the refractive index and dispersion of glasses in the BaO--BzO3-SiO 2 system. They report the values of the refractive index nD and the Abbe number v = ( n o - 1)/ (nF" -- no), from which it is possible to calculate the mean dispersion nF" - no. The values so obtained are plotted in fig. 1. There is a suggestion that the results may be represented by two straight lines with a change in slope at approximately 32 mol% BaO. In silicate systems, e.g. Na20-SiO2, the mean dispersion increases linearly with increasing alkali content. The literature on glass properties contains many observations of p r o p e r t y - c o m position relationships showing maxima, minima or changes in slope. When changes in slope are reported, there is always a suspicion that the results might equally well be represented by a smooth curve. Also, it is not often possible to relate convincingly the position of the change in slope with what is known about the structure o f the glass. For glasses in the B a O - B 2 0 3 system, Greenblatt and Bray [2] maintain, on the basis of their interpretation of NMR results, that, in the composition range below ~33 mol% BaO, the oxygen ions introduced by the BaO are taken up by the formation of BO 4 tetrahedra. At higher BaO contents, the fraction of four-coordinated borons was found to decrease. Although the situation is not absolutely clear-cut, in that the glasses studied contained between 6 and 12% silica, it is not unreasonable 231
232
D. lYarner, H. Rawson / Mean dispersion of barium borate glasses
to expect, on the basis of the results of Greenblatt and Bray, that non-bridging oxygens begin to appear beyond 33 tool% BaO. Stevels [3] and many subsequent authors have proposed that the UV cutoff in oxide glasses is associated with the excitation of electrons in the oxygen ions. It is also suggested that the excitation energy is less for a non-bridging than for a bridging oxygen ion. This explains the shift of the UV absorption edge of silicate glasses to longer wavelengths with increasing content of basic oxide. Because of the connection between absorption and dispersion, one expects the dispersion to increase with increasing content of basic oxide. One would also expect to find a more rapid rate of increase of dispersion if the basic oxide addition results in an increasing proportion o f non-bridging oxygens than if it does not. In the light of these considerations, the increase in slope of the dispersion-composition curve at about 33 tool% BaO is not surprising. However, the method used for obtaining the mean dispersions from the data of Hamilton et al. gives values the accuracy of which it is difficult to assess. The error on individual values of the Abbe number was expected to be as much as +1. Thus, the calculated mean dispersion values could be between 1 and 2% in error. The purpose of the present work was to obtain more data so as to be able to test statistically for the existence of a discontinuity in the dispersion-composition relationship.
2. Experimental A series of barium borate glasses was melted from A.R. grade chemicals in the range 2 0 - 4 0 tool%. The glasses were analysed, the results being given in table 1. A
Table 1 Glass compositions determined by analysis. mol%
1 2 3 4 5 6 7 8 9
BaO
B203
20.7 23.3 26.0 28.5 30.8 33.4 35.8 38.6 40.0
79.3 76.7 74.0 71.5 69.2 66.6 64.2 61.4 60.0
D. Warner, tt. Rawson /Mean dispersion of barium borate glasses
233
duplicate analysis was carried out on glass 9 only. These gave BaO contents of 40.05 and 40.08%. Hamilton et al. consider that the probable error in their BaO values is -+0.06 tool%. The glasses were prepared in a platinum alloy crucible and were homogenized by first bubbling oxygen through the melt and then stirring mechanically, using a motor-driven platinum blade mounted on the end of a refractory rod. The temperature was varied between 1050 and 1200°C during melting to obtain optimum homogenization and refining. The melting time was about 7 h. The melt was cast into metal moulds, care being taken to prevent folding of the glass stream as it flowed fr from the crucible into the mould. The moulds were preheated to 3 0 0 - 3 5 0 ° C , otherwise the castings cracked. The blocks were roughly annealed by placing them in a muffle furnace held at 610°C. The furnace was allowed to cool freely for 16 h to room temperature. The blocks were cut into rectangular samples suitable for refractive index measurement. After grinding and polishing these measured approximately 15 X 15 X 5 nun. They were then given a more careful anneal. [n the cylindrical tube-furnace used, each specimen was placed in a hole in a thick, circular, stainless steel plate, the holes being equidistant from the central vertical axis of the furnace. The temperature was raised to 620°C and held for 6 rain, after which the power was switched off. The cooling rate down to 500°C was almost constant at about 2.5 ° rain - l . A Pulfrich refractometer was used to measure the refractive indices for the sodium D line and the hydrogen C and F lines. At each wavelength 10 readings were made on each specimen, the specimen being removed from the instrument prism and replaced between each measurement. The optical sealing liquid used was ~-monobromonaphthalene. Care was taken to minimize the amount of liquid used so that the liquid film between the specimen and the instrument prism was as thin as possible. The density of each specimen was measured by the Archimedes method. The specimens, which were free from any seed which could be detected by eye, were weighed in air and toluene.
3. Results The density and refractive index results are given in table 2. Fig. 1 compares the mean dispersion results of the present work with those of Hamilton et al. The length of the vertical lines corresponds to two standard deviations. Regression analyses were carried out to determine whether the results could be represented by: (a) a straight line, (b) two straight lines of different slopes intersecting at 33 tool% BaO, (c) a smooth curve of the f o r m y = ax b. The results in fable 3 shows lhat there is nothing to choose between hypotheses (b) and (c). The density results are shown in fig. 2. Below 31 tool% BaO the results fall almost exactly on a straight line. Beyond this point the values fall below the line obtained by extrapolating that
234
D. Warner,H. Rawson /Mean dispersion of barium borate glasses
¢)
o
.=.
D. Warner, H. Rawson /Mean dispersion o f barium borate glasses
235
11 0 x 10 -3
10.5
/ //
ill 17 z
o
100
u') c3
95
90
8 5
210
I 25
I 30
t 35
I 40
BoO CONTENT, mol. %
Fig. 1. Relationship between mean dispersion and composition of glasses in the B a O - B 2 0 3 system. Solid circles are the results of Hamilton et al. [ 1 ] ; open circles are those from the present work.
p l o t t e d using t h e similarly. Finally, fig. 3 sition. B o t h o u r a b o u t 32 tool%
results at lower BaO c o n t e n t s . T h e refractive i n d e x results b e h a v e s h o w s the r e l a t i o n s h i p b e t w e e n m o l a r r e f r a c t i v i t y R m a n d c o m p o results a n d t h o s e o f H a m i l t o n et al. s h o w s a b r o a d m i n i m u m at BaO. T h e m o l a r r e f r a c t i v i t y was c a l c u l a t e d using t h e e q u a t i o n
R m = (/7 2 -- l ) / ( r / 2 + 2 ) .
Vm,
w h e r e F m is the m o l a r v o l u m e .
Table 3 Linear regression analysis of mean dispersion results. Correlation coefficient 1. Two straight lines intersecting at 33 mol% BaO 2. Smooth curve of the form y = ax b
0.981 0.984
D. Warner, H. Rawson /Mean dispersion of barium borate glasses
236
i
i
i
3.8
/ //
3.6
'7 ~ E
34
)FN z w o
3.2
3(
28
20
I
I
I
25
30
35
40
BQO CONTENT, tool %
Fig. 2. Relationship b e t w e e n density and c o m p o s i t i o n of glasses in the B a O - B 2 0 3 system.
990
,
r
,
i 30
L 35
\\
u >_" >
i~
985
0
9.80
9 75
i 20
i 25
i 40
BoO CONTENT, mol. % Fig. 3. Relationship b e t w e e n molar refractivity and c o m p o s i t i o n o f glasses in the B a O - B 2 0 3 system. Solid circles are the results of H a m i l t o n et al. [1 ]; o p e n circles are those f r o m the present work.
D. Warner, H. Rawson / Mean dispersion of barium borate glasses
237
4. Conclusions Although the dispersion results obtained in the present work do not provide any move support for the existence of a discontinuity in the dispersion-composition relationship than do the results of Hamilton et al.; the trends of other properties show significant changes at about 33 mol% BaO. These changes eorrelate well with changes indicated by a technique for studying local structure, i.e. NMR, which is reasonably well established. Although the molar refractivity versus composition plot provides the most obvious indication that something significant seems to be happening to the glass structure at about 33 mol% BaO, it is our opinion that, when considering how this work might be extended, it would be preferable to concentrate on the dispersion-composition relationship in that the interpretation of the phenomenon of dispersion is more straightforward than the interpretation of density and refractive index results. Also, the dispersion is a property related to the same structural feature as that detectable by NMR, i.e. the proportion of non-bridging oxygens. It would be of interest to repeat the present work using a better instrument for measuring refractive index than the one available to us, and also to study dispersion-composition relationships in other systems on which NMR studies have been carried out, e.g. the NazOB203-SiO 2 system.
References [ 1] E.H. ttamilton, G.W. Cleek and O.H. Grauer, J. Am. Ceram. Soc. 41 (1958) 209-215. [2] S. Greenblatt and P.J, Bray, Phys. Chem. Glasses 8 (1967) 190-193. [3] J.M. Stevels, Proc. 1lth Congr. Pure and Appl. Chem., Vol. 5 (1953) pp. 519-522.